U.S. patent number 4,483,268 [Application Number 06/379,350] was granted by the patent office on 1984-11-20 for method of manufacturing boat parts submerged when in use, and part produced by the method.
This patent grant is currently assigned to Volvo Penta AB. Invention is credited to Heinz Pichl.
United States Patent |
4,483,268 |
Pichl |
November 20, 1984 |
Method of manufacturing boat parts submerged when in use, and part
produced by the method
Abstract
A method of manufacturing boat parts submerged when in use
comprises producing a metallic skeleton structure shaped for
easiest production by conventional metal-forming methods, and
covering this structure in a mold with an adhering layer or coating
of a thermoplastic resin or a rubber-based vulcanizable substance.
The inner walls of the mold have high finish and the desired
streamlined outer shape of the finished product, such as a lower
unit of a propulsion system, or a propeller and the like. The
coating functionally neutralizes the technologically conditioned
and generally not streamlined shapes of the skeleton structure and
provides a protection of the skeleton structure against
electroerosive corrosion.
Inventors: |
Pichl; Heinz (Upsala,
SE) |
Assignee: |
Volvo Penta AB (Gothenburg,
SE)
|
Family
ID: |
20343891 |
Appl.
No.: |
06/379,350 |
Filed: |
May 18, 1982 |
Foreign Application Priority Data
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May 21, 1981 [SE] |
|
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8103204 |
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Current U.S.
Class: |
114/65R;
264/271.1; 264/273; 416/229R; 416/241A; 440/49; 440/78 |
Current CPC
Class: |
B29C
37/0085 (20130101); B29C 70/70 (20130101); B63H
1/14 (20130101); B63B 5/24 (20130101); B29C
45/00 (20130101); B29K 2021/00 (20130101); B29L
2031/3067 (20130101); B29L 2031/749 (20130101); B29K
2995/0058 (20130101) |
Current International
Class: |
B29C
37/00 (20060101); B29C 70/00 (20060101); B29C
70/70 (20060101); B63H 1/14 (20060101); B63B
5/24 (20060101); B63B 5/00 (20060101); B63H
1/00 (20060101); B29C 45/00 (20060101); B63B
005/24 () |
Field of
Search: |
;114/65R,357,74,79
;264/270,271,273 ;416/241A,229,230 ;440/71,76,77,78 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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124253 |
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Mar 1919 |
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GB |
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412300 |
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Jun 1934 |
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GB |
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Primary Examiner: Blix; Trygve M.
Assistant Examiner: Young; Patrick W.
Attorney, Agent or Firm: Yeager; Arthur G.
Claims
What is claimed is:
1. A submersible lower unit of a marine propulsion assembly,
comprising a rigid inner metallic skeleton non-streamlined outer
surface structure including a first pressure-resistant body
extending generally vertically between an upper end and a lower end
opening and defining a generally cylindrical first inner cavity, a
second pressure-resistant body extending generally horizontally
between a forward end and a rear end opening and defining a
generally cylindrical second inner cavity, said bodies and cavities
being connected one with another at a place of interconnection at
their said lower and forward ends and accommodating power
transmission means drivable by a motor unit located adjacent said
upper end, said power transmission means comprising generally
vertical, rigid drive shaft means located in said first cavity,
generally horizontal rigid drive shaft means located in said second
cavity, and a bevel gear operatively interconnecting said vertical
and horizontal drive shaft means and located at said place of
interconnection, driving propeller means mounted on said horizontal
driving shaft means aft of said rear end, and an outer coating of a
watertight, dielectric substance on the said inner skeleton
non-streamlined outer surface structure, which coating tightly
envelops said structure and defines an outer final shape and face
which is streamlined throughout said lower unit.
2. The unit of claim 1 further comprising at least one fin element
projecting outwardly of at least one of said first and second
bodies and fully embedded in said coating.
3. The unit of claim 2, wherein said fin element is provided with
mechanical means for enhancing adherence of said coating thereto
and including at least one opening therethrough.
4. The unit of claim 2, wherein said fin element is provided with
mechanical means for enhancing adherence of said coating thereto
and including at least a reinforced edge along the free edge of
said fin element.
5. The unit of claim 1, wherein a connecting flange for attachment
of said skeleton structure to said motor unit is provided around
said upper end opening.
6. The unit of claim 1, further comprising a third body defining a
cylindrical cavity for a duct or for accommodating additional
mechanical components and which extends parallel with at least one
of said first and second bodies and is rigidly connected to said
skeleton structure.
7. The unit of claim 1, wherein said skeleton structure includes a
cooling water intake.
8. The unit of claim 1, further comprising a removable cover means
releasably attached to said forward end to provide access to said
bevel gear.
9. The unit of claim 8, wherein said cover means includes heat
conducting material uncoated by said substance to transmit heat of
said bevel gear to the surrounding water.
10. The unit of claim 1, wherein around at least a portion of the
edge of at least one of said openings said substance forms a
circumferential annulus covering at least partially the total
breadth of said edge, said annulus enhancing the adhesion of said
coating substance thereto and providing sealing means for
connecting another part thereto.
11. The unit of claim 1, wherein said coating substance is a
thermoplastic resin.
12. The unit of claim 11, wherein at least one layer of a primer
substance is provided on said skeleton structure for enhancing the
adhesion of said coating substance thereto.
13. The unit of claim 12, wherein said coating substance is a cured
rubber-based substance.
14. The unit of claim 12, wherein said coating substance includes
an additive for rendering said outer coating a cathode.
Description
BACKGROUND OF THE INVENTION
The invention relates generally to a method of manufacturing boat
parts submerged when in use (i.e. so called "wet" or underwater
parts) such as rudder plates, propellers, lower units of outboard
motors and of inboard-outboard motors etc., and to parts
manufactured by the method.
Conventionally, such parts are produced of metallic materials such
as iron, zinc or aluminum by molding or diecasting and they are,
depending on the material used, provided with several surface
conditioning layers of varying character, such as zinc coatings,
chromating layers, primer layers, paint coatings etc. To apply a
plurality of such layers is expensive, among other things also
because relatively long waiting times may be necessary between two
subsequent applications, to allow the underlying layer first to dry
properly.
However, in spite of the presence of a plurality of surface
conditioning layers, it cannot be excluded that after a certain
time of service, or when hitting an underwater obstacle, a defect
or fault arises exposing a bare metallic surface to water. Thereby
arises the risk of corrosion by an electroerosive process taking
place between the exposed spot and some other submerged bare
metallic part, even if the involved metals may not be susceptible
to corrosion by oxidation. Not even sailing boats with hulls of
wood or plastics are exempt, because they mostly have keels
comprising ballast metal (iron) and electroerosion may occur
between this metal and e.g. an auxiliary outboard motor made mainly
of aluminum.
It will be readily understood that submerged parts of a boat must
be as much streamlined as possible. Said conventional surface
conditioning layers, irrespective of their character and number,
can always create only a uniformly thick coating on the underlying
metallic surface, so that this surface in general must have the
final shape of the respective part called for by hydrodynamic
reasons, i.e. a streamlined shape. Moulding and diecasting
processes do not allow greater differences in wall thickness (and
such differences in a metallic object would also cause a
considerable increase in weight) so that the inner walls of all
underwater parts having a cavity, such as the lower units of
outboard or inboard-outboard drive units, must generally follow the
streamlined shape of the outer walls. Because of this, relatively
complex mandrels or mold cores, often more than one, must be used
in order to obtain the necessary streamlined outside shape of a
casting which then may be covered by uniformly thick surface
conditioning coatings.
Attention is directed to German patent specification No. 360952,
issued Oct. 9, 1922, to Zeppelin-Werke G.m.b.H. and Albert Lehrle,
and which discloses a settable ship's propeller of aluminum, and to
Swiss patent specification No. 527680, issued Sept. 15, 1972, to
Societe Nationale Industrielle Aerospatiale, which discloses a
method of covering a structure having a cavity with a layer of a
liquid substance capable of being hardened.
SUMMARY OF THE INVENTION
There is provided in accordance with the invention a method that
solves the problems discussed above. This is accomplished by
producing by conventional metal-forming techniques such as molding,
diecasting or welding a metallic skeleton structure having an outer
shape, defined by its outer walls, which is fully included in the
final shape of the respective boat part, but which differs
therefrom not only by smaller dimensions, but mainly by the fact
that it in greatest possible extent is adapted to answer to the
demands of convenient and economic production by e.g. molding or
diecasting, generally disregarding all other aspects, including
streamlining. In particular in the case of parts such as the lower
units of marine propulsion assemblies which have an internal
cavity, economic and convenient production demands that only simple
and few cores are needed to be used, e.g. only one or two
cylindrical cores. With a view to the already mentioned necessity
to have in a metal cast uniformly thick walls, also the outer walls
of the cast will then be more or less cylindrical. The general
shape of a hollow cylinder has further the advantage of being
highly resistant to outer pressure, as it well may be defined as
being constructed according to known principles of shell
construction. The importance of this fact in the present context
will become clear later. Even when the skeleton structure is
produced by welding, the shape of one or more combined hollow
cylinders is technologically advantageous.
This skeleton structure is thereafter inserted into a mold as if it
were a core itself and a curable watertight and dielectric mass,
being either a thermoplastic resin or a vulcanizable rubber-based
substance, is introduced into the mold to fill out the empty spaces
between the outer walls of the skeleton structure and the inner
walls of the mold. Said inner walls are smooth (e.g. highly
polished) and have generally (i.e. with due respect to possible
shrinkage) the final shape of the manufactured part. The mass is in
the mold cured in known manner, i.e. by lapse of time and change of
its thermal condition (cooling or heating), so as to form a
relatively thick layer or coating enveloping the structure and
adhering thereto.
Emphasis has to be laid on the choice of the layer-forming
material. Both thermoplastic resins and vulcanizable rubber-based
substances have considerably shorter curing times (in the order of
magnitude of minutes) than what the case is with thermosetting
resins, and have at the same time dielectric qualities which are
fully satisfactory from the point of view of preventing
electroerosive corrosion.
However, an arbitrary skeleton structure with a cavity, e.g. the
one shown in FIG. 1 in the above cited Swiss patent specification
No. 527.680, might be destroyed by the relatively high pressure
prevailing in the mold, if not special precautions were taken to
prevent it. In the case of a thermoplastic resin, it is injected
into the mold in a heated state and under a pressure of more than
150 kg/cm.sup.2. In the case of a rubber material, pressure is
developed in the mold during the curing process. According to the
present invention, a skeleton structure having a cavity is either
constructed in accordance with known shell construction principles,
e.g. in the general form of one or more hollow cylinders, and/or
the cavity is filled with some suitable and readily removable
pressure resistant substance, liquid, sandy, or firm, e.g. a
possibly modified duplicate of a core used in the manufacture of
the skeleton structure.
Propellers made all of plastics material (with the exception of the
hub portion) have already been proposed, but they are extremely
difficult to produce in correct shape and pitch, due to contraction
of the plastics material in the blades at and after curing.
According to the present invention, the metallic skeleton structure
reduces the amount of thermoplastic resin or vulcanizable
rubber-based substance to be used, and thus also the degree of
shrinkage, and further supports the coating during the period of
curing and even thereafter. Moreover, a skeleton structure of one
single type may be adapted for insertion in different molds to
produce propellers with somewhat differing pitch and/or overall
dimensions.
The dielectric covering layer according to the present invention
will mostly have significantly varying thickness from one place to
another due to the marked discrepancy in shape between its inner
face, corresponding to the outer wall of the skeleton structure,
and the outer face shaped by the inner walls of the mold. It will
however even at the thinnest place in general be thicker than a
conventional plurality of surface conditioning layers.
Other objects and advantages of the invention will become known by
reference to the following description, claims and the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows diagramatically a sailing boat with a ballast keel and
which is provided with an inboard-outboard motor having a lower
unit manufactured according to the present invention,
FIG. 2 shows at a greater scale and partly in longitudinal
cross-section the posterior part of said inboard-outboard motor of
FIG. 1,
FIG. 3 shows in a perspective view and at still greater scale a
metallic skeleton structure for the lower unit of the motor
according to FIGS. 1 and 2,
FIG. 4a is a cross-section through the lower unit or rig of FIG. 3
along plane IV--IV of FIG. 3,
FIG. 4b is a cross-section analogic to FIG. 4a through a modified
embodiment, plane IV--IV of FIG. 3,
FIG. 5 is an exploded view of another embodiment of a lower unit
according to the invention,
FIG. 6 shows one half of a mold with a finished lower unit shown in
longitudinal cross-section,
FIG. 7 shows at a greater scale a detail VII from FIG. 6,
FIG. 8 shows a part of a skeleton structure of a propeller of FIG.
1 according to the present invention,
FIG. 9 is a cross-section along the plane IX--IX of FIG. 8, and
FIG. 10 is a cross-section through another embodiment of a lower
unit or rig according to the present invention.
Parts having identical or analogous functions are in all drawing
figures provided with identical or analogous reference
characters.
DETAILED DESCRIPTION OF THE INVENTION
According to FIG. 1, a sailing boat A has a hull 10 with a bottom
10a and a keel 11 comprising a ballast portion 11a of iron. The
boat is provided with an inboard-outboard motor 13 of the so called
sailing boat type (S-type), i.e. a motor the lower unit 12 of which
is not attached to the transom of the boat, but penetrates through
the bottom 10a of hull 10 in front of transom 10b. Said lower unit
is to a great extent manufactured of aluminum material. Although as
well ballast portion 11a, as lower unit 12 conventionally are
coated wiht paint, it will be understood that the paint coating
easily can be damaged so that metal becomes exposed and
electroerosion may occur, whereby the lower unit 12 of aluminum,
much more expensive than the iron ballast 11a, functions as an
anode and is consumed by corrosion. Voltage values up to 90V have
been measured in similar cases, and it is customary to provide
special "offering anodes" to which electroerosion should
concentrate in order to save more valuable parts from the
attack.
In FIG. 2 is shown in detail how the lower unit 12 is constructed
in accordance with the present invention. It comprises a metallic
skeleton structure 20 which is covered by or coated with a layer or
coating 27 of thermoplastic resin or vulcanized rubber-based
substance. This coating will be discussed more in detail in
connection with the following drawings.
According to FIG. 2, the inboard-outboard motor 13 comprises a
motor unit 13a, an upper bevel gear 14 in a housing 14b, an inboard
part 12a of the lower unit 12, the lower unit 12 proper placed
outboard, a power transmitting shaft 14a, a lower bevel gear 15, a
double propeller shaft 15a and two pusher propellers 16a, 16b
rotating in opposite directions. Parts 14a, 15, 15a and such parts
as exhaust conduct (not shown) etc. are accomodated within an inner
cavity 21b', 21b" in the skeleton structure 27.
In FIG. 3 the skeleton structure 20 is for clarity shown covered by
coating 27 only in its lower half, and the coating itself is shown
as if it were transparent, which in practice generally will not be
the case. According to FIGS. 3 and 4, the upper part of the
skeleton structure 20 has essentially the shape of a vertical
hollow cylindrical body 21 with an inner cavity 21b' and with an
upper flange 21a for attachment to the aforesaid part 12a which in
its turn is affixed to the housing 14b of the upper gear 14. A
cooling water intake 24 may be arranged on the outside of structure
20 and also covered by the coating. From the cylindrical body 21
protrude forwardly and rearwardly fin elements 22 and 23 which have
reinforced thickened edges 22a, 23a and are provided with a
plurality of through-openings 22b, 23b. Said edges and openings
define mechanical means promoting the adherence of coating 27 to
structure 20, as will be readily recognised from the study of FIGS.
4 and 6.
The lower part of structure 20 is essentially defined by a
horizontal hollow cylindrical body 21c attached at right angles to
the lower end of the vertical cylindrical body 21. The cylindrical
bodies 21, 21c comprise or envelop cavities 21b', 21b".
It will be readily understood that such a metallic skeleton
structure 20 may be readily produced by conventional methods, such
as by welding together one annular, two cylindrical, and two planar
pieces defining respectively the flange, the bodies comprising the
cavities, and the fin elements. It may also just as readily be
manufactured by conventional molding or diecasting method with the
use of only two cylindrical cores defining said cavities 21b',
21b".
From FIG. 4a is best apparent how coating 27, having markedly
varying thickness, levels up the outer shape of structure 12,
established with a view only to most economical production, to an
overall outer shape defined by an outer face 12b which is strictly
streamlined.
In FIG. 4b is shown a modification of the embodiment according to
FIG. 4a with a second generally vertical hollow cylindrical body
21', parallel with and adjacent to cylindrical body 21. Cylindrical
body 21' may serve e.g. as a duct for cooling water or exhaust
gases or to accomodate mechanical parts etc.
It will be appreciated that also a plurality of essentially
horizontal hollow cylindrical bodies, parallel with and adjacent to
body 21c may be provided. There may also be provided one or more
cylindrical bodies, in particular when the skeleton structure is
manufactured by welding technique which are more or less arcuate
having e.g. the upper end parallel with the vertical cylinder and
the lower end parallel with the horizontal cylinder, and serving as
ducts for water or exhaust gases.
Cylindrical body 21 is located essentially at the thickest portion
of coating 27 (maximum thickness f) where also smallest wall
thickness c of the coating is to be found. However, even this
minimum thickness c is a multiple of the thickness of the
aggregated conventional surface conditioning layers. The skeleton
structure 20, and in particular the cylindrical bodies 21, 21c have
generally uniform wall thickness all over. In FIG. 4a is also shown
power transmitting shaft 14a which of course will be mounted first
upon assembly of the whole propulsion system.
A considerable amount of heat is generated in operation in the two
bevel gears 14, 15, and coating 27 insulates the structure 20 not
only electrically, but also thermically. The upper gear 14 is
cooled in conventional manner, not shown, by water fed-in through
intake 24. As to the lower gear 15, reference is made to FIG. 5,
where another embodiment of lower unit 12' is shown. There the
front part of lower unit 12', or more precisely of its lower
horizontal cylindrical body 21c', is left uncovered by the coating
27 and is preferably constructed as a removable cover means 16
whereby also access to the inner space 15b at the front of cavity
21b", where the lower gear 15 is located, is enabled. The cover
means 16 is made of heat conducting material. Such material may
also be metallic, e.g. an aluminum-bronze alloy or copper. In view
of the relatively small amount of material needed for the cover
means, more expensive, corrosion-proof material may be chosen, or
alternatively, the cover means may constitute the earlier mentioned
offer anode. Protecting guards 24a are to be mounted at the water
intake 24.
According to FIG. 6, a mold for manufacturing a boat part according
to the present invention may comprise two essentially identical
halves such as half 30, firmly attachable one to another by
convenient affixing means inserted into openings 30a. A feed-in
channel 31 is provided in at least one of the halves for the supply
of the material forming the coating 27 ("layer material").
Skeleton structure 20 is immediately after production (by welding,
molding or diecasting) sand-blasted and provided with a ground
coating e.g. by immersion in paint or by electrostatic application
of a plastics layer on the heated skeleton structure. One or more
primer layers, e.g. of an epoxy resin, may be applied to the
surface of the skeleton structure which have such composition that
they react with the layer material and bond it to the skeleton
structure. Such a primer layer defines a chemical means promoting
adhesion of the layer to the skeleton structure and may be obtained
e.g. by coating the surface of the heated skeleton structure with a
plastics powder which firmly adheres thereto and later on reacts
with the layer material. If the layer material is a rubber-based
substance, the skeleton structure is also sand-blasted, but
possibly not provided with a ground coating. Appropriate
rubber-based materials are e.g. chloroprene rubber (neoprene) and
nitrile rubber, which both have good aging resistance.
Then connection faces such as edge 20a (FIG. 7), discussed more in
detail later on, may be machined, and thereafter the skeleton
structure 20 is inserted into the mold and is "suspended" there
with the aid of sealing cores such as cores 32-34 sustaining
structure 20 at places where the finished product will have
openings communicating with the outside. The skeleton structure 20
of FIG. 6, consisting essentially of two cylindrical bodies, may be
considered to be constructed in accordance with shell construction
principles, and basically no further steps are necessary to protect
it against the pressures which will develop in the mold. However,
for clarity, the entire inner cavity 21b', 21b" is shown to be
filled with a sandy mass 35, e.g. well shaked sand, filled in
previously and now firmly plugged by said cores 32 to 34 which in
their turn are secured in the mold in a manner not particularly
shown in the drawing.
The layer material is fed-in through channel 31 and fills out all
empty spaces around the suspended skeleton structure, inclusive the
openings 22b, 23b in the fin elements 22, 23, and envelops also the
thickened edges 22a, 23a.
A third fin element 28 with a thickened edge 28a and openings 28b
is attached to the bottom part of structure 20.
Appropriate thermoplastic resins are e.g. soft PVC, which has a
certain rubber-like character, nitryle PVC, or thermoplastic
polyurethane, which is very resistant to wear even in respect of
scratch and impact resistance, or a rubber-thermoplastics-alloy
based thereon. Thermoplastic resin material is fed-in into the mold
heated and under pressure of more than 150 kg/cm.sup.2. Pressure
values of appr. 500 kg/cm.sup.2 are normal and up to 900
kg/cm.sup.2 are possible. Temperatures between 150.degree.
C.-280.degree. C. may occur. A coating of thermoplastic material
has the advantage over a coating of rubber-based material that it
readily may be repaired with the aid of an appropriate cement
(solvent).
If a rubber-based coating is to be produced, the substance is
fed-in into the mold in cold state and is warmed therein to be
cured or vulcanized during e.g. 20 minutes. Pressure values over
150 kg/cm.sup.2 develop in the mold during the curing process.
As well to thermoplastic as to rubber-based material additive
substances may be added, e.g. pigments to obtain coloration, or
materials effective as a cathode, such as carbon.
From the study of FIG. 7 will be apparent that the edges around the
openings in skeleton structure 20, such as edge 20a, may
advantageously be somewhat retracted, at least along a portion B'
of their total breadth B, and be provided with a groove 20b so that
the coating will form there a circumferential annulus 25 terminated
by a bead 25a. Annulus 25 may of course also cover the whole
breadth B, i.e. edge 20a of the structure 27B is along its entire
breadth B retracted relative the sealing core 33, and constitutes a
sealing means in respect of a connected part which e.g. may be
affixed with the aid of a thread 26 in the inner wall of structure
20. Parts 20a, 25 and 25a define another mechanical means promoting
adherence of the coating 27 to the structure 20.
After the layer material which has been fed-in into the mold has
cured, by lapse of time and cooling in the case of thermoplastic
resin, or by lapse of time and heating in the case of a
rubber-based material, the finished product is removed from the
mold. It will be perfectly finished on the outer surface, in
accordance with the quality of the inner walls of the mold, and it
will possibly also have a desired color.
It will be understood that the measures taken to protect the hollow
skeleton structure against pressure in the mold, i.e. shell
construction and/or filling up of the cavity, also are effective to
neutralise the crimping pressure generated when coating 27 finally
sets. This unavoidable crimping guarantees adherence of the layer
to the structure even if no additional mechanical or chemical means
were used.
In FIG. 8 is shown propeller 16a as an example of a boat part
having a skeleton structure without any cavity. Conventionally,
boat propellers are made of steel or other metals, inclusive
aluminum. Aluminum propellers however cannot be used for speeds
over appr. 50 km/hour due to cavitation effects. Plastics are
practically not susceptible to cavitation, but as already stated,
such propellers are rather instable in respect of shape. Therefore,
steel propellers have conventionally been accepted as the best
compromise with a view to cost and performance.
According to the present invention, a metallic skeleton structure
40, preferably of steel, is provided. Structure 40 has in the
region of each propeller blade such as blade D a plurality of
generally radially extending rib elements 41 which are attached to
a hub or central portion 44. (Propeller blade E is shown in
finished state.) Between and outside the ribs 41 extend
plate-shaped portions 41a provided with a plurality of
through-openings 40a having the same purpose as similar openings
22b, 23b in FIGS. 2, 3 and 6, i.e. to allow passage for the layer
material when injected into the mold. It will be recognised from
the drawing that the area limited by the outline of metallic
skeleton structure 40 occupies at least 70% of the area of the
respective propeller blade D. The interspaces between the rib
elements 41 may also be left free, but it is essential that they
always are at least partially penetrable for the layer material, as
the case is e.g. in FIG. 8 thanks to openings 40a.
The manufacture is the same as described in connection with FIG 6.
The peripheric contours 45 of the finished blade D are shown in
phantom in FIG. 8 and with full lines in FIG. 9.
It is possible to use the same skeleton structure 40 for several
propeller types which in minor degree differ in diameter and/or in
pitch. Slight differences in pitch are the case e.g. with two
propellers 16a, 16b in a double propulsion unit as shown in FIGS. 1
and 2. The same skeleton structure is inserted in different molds
which of course exactly correspond to the desired final propeller
shapes. It will be recognised from the phantom outline in FIG. 8
that coating 27' covers as well the blades, as the outside of the
central hub portion of the propeller. The skeleton structure 40 may
be readily mounted in the mold e.g. with the aid of a mandrel
inserted into the opening of the hub portion instead of a future
propeller shaft.
It will be readily realised from the drawings that the metallic
skeleton structure always has an outer shape, defined by its outer
walls, which is fully comprised within the outer shape of the final
product, even if the skeleton structure possibly at some place
itself defines a part of the outer shape, as is the case e.g. with
the innermost portion of edge 20a in FIG. 7, not covered by annulus
25.
In FIG. 10 is shown more in detail an exemplary embodiment of a rig
or lower unit according to the present invention. The unit 12' is
provided for a double propeller drive and has a double propeller
shaft 15a accomodated in the cavity 21b" defined by the horizontal
hollow cylindrical body 21c. Where the vertical cylindrical body 21
and said body 21c meet is the bevel gear 15 arranged. A second
vertical cylindrical body 21' is parallel with and adjacent to body
21. In body 21' is accomodated an intake duct 121 for cooling water
entering through ports 24' and in said duct is concentrically
mounted a duct 122 for lubricating oil. Coating 27' has been
produced at the same time as layer 27 by inserting into body 21' a
core of smaller diameter than what the diameter of the cavity
defined by body 21' is.
Cover means 16' provide access to gear 15 at assembly and
disassembly and can be of metal or plastics.
* * * * *